The present invention relates to a satellite television broadcasting receiver to be used for receiving satellite television broadcasting in accordance with a communication satellite or a broadcasting satellite.
In recent years, communication services utilizing communications satellites (CS) have been extensively provided. A satellite television broadcasting receiver which is used for the purpose of receiving pictures and television broadcasting programs offered by utilizing satellites has basically the same structure as that of the receiver for receiving television broadcasting programs according to a broadcasting satellite, as shown in FIG. 3.
Referring to FIG. 3, 1 designates an input terminal for inputting a first intermediate frequency signal. The input terminal 1 frequency converts radio waves of a 12 GHz band which have been received from a satellite by an antenna into a frequency of 1 GHz band by a down converter, and applies a first intermediate frequency signal guided indoors by a coaxial cable. 2 designates a channel selecting circuit for receiving signals of one channel by selecting one broadcasting station out of multiple of first intermediate frequency signals. 3 designates a band-pass filter for passing only one wave out of output signals of the channel selecting circuit 2. 4 designates an FM demodulator for demodulating an FM signal, which is an output signal of the band-pass filter 3. 5 designates a deemphasis circuit for deemphasizing a demodulator output signal which is an output of the FM demodulator 4. 6 designates a low-pass filtering circuit for passing therethrough only a video signal. 20 designates a first amplifier for amplifying the output signal of the low-pass filtering circuit 6. 21 designates a clamping circuit for removing an energy dispersion signal included in the output signal of the first amplifier 20. 22 designates a second amplifier for amplifying the output of the clamping circuit 21. 12 designates a video signal output terminal for outputting the output signal of the second amplifier 22. 13 designates an audio signal processing circuit for outputting an audio signal from a demodulator output signal which is an output of the FM demodulator 4. 14 and 15 designate audio signal output terminals for outputting output signals of the audio signal processing circuit 13.
The operation of the satellite television broadcasting receiver having the above-described structure will be explained below. A first intermediate frequency signal, guided indoors by a coaxial cable, which has been obtained by frequency converting the waves of 12 GHz band received by the antenna from the satellite into the frequency of 1 GHz by the down converter, is applied to the input terminal 1. By selecting one broadcasting station by the channel selecting circuit 2, signals of only one channel are received out of multiple first intermediate frequency signals. Out of the signals selected through the channel selection by the channel selecting circuit 2, only one wave is passed by the band-pass filter 3. A SAW filter is generally used for the band-pass filter 3. The FM demodulator 4 demodulates the FM signal of the selected channel, to obtain a demodulator output signal. The audio signal processing circuit 13 demodulates an audio signal which has been QPSK modulated in the subcarrier of 5.7 MHz from the demodulator output signal. After PCM demodulation of the signal, the audio signal processing circuit 13 converts a digital signal into an analog signal by a D/A converter, and outputs the audio signal to the audio signal output terminals 14 and 15 respectively through a low-pass filter. The demodulator output signal of the FM demodulator 4 is inputted to the deemphasis circuit 5 to flatten the frequency characteristics of the signal that has been preemphasized by the transmitter. The signal that has been flattened by the deemphasis circuit 5 is inputted to the low-pass filtering circuit 6 so that only the image signal is passed and outputted by the low-pass filtering circuit 6.
The spectrum of the FM demodulator by the video signal becomes such that energy is concentrated around the frequency corresponding to a pedestal level or a synchronizing signal level which have a large time factor in the video signal level, having a peak of electric power in this energy concentrated region. Since this peak interferes with the microwave circuits, such as telephone lines, the WARC-BS has prescribed that the power flux density of satellite broading waves is lowered by 22 dB in the band width per 4 KHz. This is called energy dispersion which is achieved by superposing a triangular wave synchronous with the frame frequency on the video signal. In the case of a satellite television broadcasting according to a broadcasting satellite, the frequency shift of the energy dispersion signal is 600 KHzp-p and the repetition frequency of the energy dispersion signal is 15 Hz. Since the frequency shift of the main carrier is 17 MHzp-p, the level of the energy dispersion signal which is superposed on the video signal 1 Vp-p becomes 0.11 Vp-p when the preemphasis applied to the video signal is taken into account. FIG. 4 shows the state of the video signal on which the energy dispersion signal has been applied in the case of television broadcasting according to a broadcasting satellite. (reference document: A Satellite Broadcasting Receiver (Part 2, desirable performance); The radio engineering and electronics association).
In the meantime, in recent years, there have been various schedules for carrying out television broadcasting by using communications satellites. In this case, severer conditions have been prescribed for these communications than those for a satellite broadcasting. In the case of a television broadcasting in Japan based on a communications satellite, the frequency shift of the energy dispersion signal is in the range from about 2 MHzp-p to about 3 MHzp-p and the repetition frequency of the energy dispersion signal is a triangular wave of 30 Hz. Since the frequency shift of the main carrier is different depending on the satellite, the level of the energy dispersion signal superposing on the video signal 1 Vp-p is about 0.5 Vp-p at maximum when the preemphasis applied to the video signal is taken into account. FIG. 5 shows a state of the video signal on which the energy dispersion signal is superposed in the case of television broadcasting according to a communications satellite.
On the image signal which has been FM demodulated, deemphasized and passed through the low-pass filter, the energy dispersion signal has been kept superposed. When the video signals are reproduced on the screen in this state, a flickering interference occurs on the signal.
When the output signal of the low-pass filtering circuit 6 is applied as an input, the first amplifier 20 amplifies the signal to 2 Vp-p in the level of the image signal excluding the portion of the energy dispersion signal and produces the result as an output. The clamping circuit 21 eliminates the energy dispersion signal which has been superposed on the video signal and applies a DC bias to the second amplifier 22. The second amplifier 22 is a buffering circuit in which the voltage gain is 1. A DC bias of satisfactory DG and DP is set by the clamping circuit 21 and the video signal of 1 Vp-p is outputted in the terminal value of 75.OMEGA. from the video signal output terminal 12.